Field of the Invention
The present invention relates to a method and a system for removing motion artefacts in Optical Coherence Tomography.
Description of the Related Art
Optical Coherence Tomography (OCT) is an established medical imaging technique that uses coherent light to capture tomographic images of a subject. In recent times, Angiography (OCTA) is developed as a desirable non-invasive imaging technique for generating volumetric angiography images of a fundus of an eye.
Non-patent literature 1 and non-patent literature 2 discuss a common method of OCTA image generation based on amplitude decorrelation algorithms that are applied to a number of images acquired from the same position of an eye. Any blood flow in the tissues (e.g. blood vessels (including capillaries)) causes a signal intensity variation, while other (stationary) tissues without any blood flow do not cause any intensity variation of the signal. Thus, the amplitude decorrelation processing detects the intensity variation and emphasizes the blood flowing (moving) within the fundus.
Although the number of images at the same position of an eye are taken in quick succession, movement of the eye between and/or during each image taking can still occur. Any movement of the eye between repeated images (B-scans) during OCTA examination can cause a significant increase of artefacts in the decorrelation signal (i.e. differences between the images). This is because the repeated images will be different from each other, and since the amplitude decorrelation algorithm detects and emphasizes differences in signal intensity regardless of the source of such changes, all changes including different positions of tissues other than blood vessels will be detected.
In order to perform OCT and OCTA a measurement scanning beam from an OCT apparatus is directed into one of the eyes of a patient. The patient's eye movements in relation to the scanning beam can be divided into two kinds, namely eye-induced movements and head-induced movements. Eye-induced movements are rapid and can significantly change the position of the retina against the scanning beam in a transverse direction. Head-induced movements, however, are not rapid and can be considered as a kind of drift caused by breathing, blood pulses, or the general condition of the patient. Head-induced movements may occur in a transverse direction (e.g. side-to-side with respect to the scanning beam) or a longitudinal direction (e.g. along the direction of the scanning beam).
Transverse head movement and longitudinal head movements have a different influence on the OCT tomogram. FIG. 1 shows these differences. Part A of FIG. 1 shows at the top a diagram indicating transverse head movement, and at the bottom the resultant tomogram. Part B shows at the top a diagram indicating longitudinal head movements, and at the bottom the resultant tomogram.
As FIG. 1 shows, transverse head movements (part A) produce a tilt of the retinal signal on the corresponding OCT tomogram. This is because movements in the XY plane influence the position of a pivot of the scanning beam, and cause a path length change along the X or Y axis. It can be seen that such transverse movement does not necessarily change the average position on the Z-axis (as shown). In the case of longitudinal head movements (part B), the average position on the Z-axis varies significantly, but the retinal signal tilt is relatively maintained. In a typical case, a combination of both types of head movement is present during OCT measurement.
Head-induced movements, although not rapid, may significantly degrade the OCTA image quality. This is due to the high sensitivity of the amplitude decorrelation based methods used to generate the motion contrast image (e.g. angiogram). It is well known that OCT structural images are disturbed by speckle noise in which the axial size of the speckles is comparable to the axial optical resolution of the OCT apparatus, and the transverse size of the speckles is given by the spot size of the scanning beam OCT apparatus at the object. In a high-resolution OCT apparatus, the axial resolution may be as low as 3 μm, while the transverse resolution is between approximately 15 to 25 μm and is limited mainly by the optical properties of the human eye. Hence, any phenomenon which can modify two tomograms in the Z-axis at the level of half the size of a speckle may cause a significant increase of the decorrelation signal.
For example, consider the case of an OCT apparatus that has an acquisition speed of 70,000 kHz, obtaining two images consisting of 300 A-scans each of which will be analysed for motion contrast. The time interval between obtaining the two images will be approximately 5 ms. Decorrelation will be significantly disturbed when a shift between the two images in the Z-axis is half of a speckle, i.e. ½ of 3 μm. Thus, a speed of eye movement that would significantly disturb the motion contrast image will be approximately 1.5 μm/5 ms=0.3 mm/s. Such eye movement could easily be achieved by processes such as breathing, or the inaccuracy of head fixation of a patient while performing the measurement.
FIG. 2 shows a 3×3 mm region of an in vivo OCTA projection map influenced by eye movements during the acquisition (measurement). Specifically the patient's eye movements are along the Z-axis, and the influence on the OCTA projection map can be clearly seen as (white) horizontal lines.
To obtain such an image, two mirrors (e.g. galvanometric driven mirrors) are required to manipulate a scanning beam in two transverse directions X and Y. For raster scan pattern, typically the X-mirror performs scanning along the X-axis to obtain a single B-scan and the Y-mirror shifts the beam to a new location along the Y-axis to collect the next B-scan. However, an angle can be applied to the scanning pattern meaning that the single B-scan is acquired by manipulating both the X and Y mirrors. Hence, hereinafter, a fast axis will be discussed as an axis given by direction of acquiring a single B-scan, and a slow axis will be discussed as an axis given by the direction of acquiring another B-scan or B-scan sets.
There are two known strategies or methods aimed at reducing the influence of eye movements on OCTA projection maps. These two methods of OCTA processing for a single B-scan set are illustrated in FIG. 3 as method A on left and method B on the right.
Method A will be described first. In step 1 the sets of B-scans are acquired. Depending on the OCTA technology used, each B-scan can be repeated several times (minimum once). After acquisition of the B-scans, the tomogram reconstruction process is applied to each B-scan (step 2).
In step 3 an alignment process is applied within a single set of B-scans independently for each set of B-scans. In other words, for a set of B-scans obtained at a particular slow-axis position (X or Y position) (B-scan set), the alignment process is applied for that set independently from any other B-scan set. The purpose of this step is to compensate for small displacements between the B-scans caused by eye motions during the acquisition.
In step 4, the angiograms are calculated for each pair of adjacent B-scans. In step 6 the final angiogram is calculated by, for example, a combination of the angiograms calculated in step 4 (e.g. by using maximum image projection, average or median or any other combination).
Method B differs from method A by omitting step 3 and including an additional step 5. Because step 3 is not present in method B, the angiograms are calculated for each pair of B-scans in step 4 regardless of whether the B-scans are corrupted, or influenced, by eye movements. However, method B includes the additional step 5, in which the angiograms corrupted by eye movement are removed. The final angiogram is calculated in step 6 as a combination of the remaining angiograms.
The main difference between method A and method B is the way of dealing with eye movements. Method A tries to compensate for eye movements in post-processing, whereas method B removes the angiograms influenced by eye movements. Method B was described in non-patent literature 1.
In method B, step 5 requires the determination of a threshold value which is applied to decide whether a tomogram is influenced by eye movements, and consequently, whether to delete the tomogram or not. This way it can efficiently remove artefacts caused by significant eye movements and can therefore reduce some strong artefacts in the final angiogram. However, in the case of small eye movements, the determination of a robust threshold may not be possible because such a threshold would be at the level of a useful signal, and therefore angiograms not influenced by eye movements could be removed. Another problem with method B is that by removing the influenced angiograms, the number of angiograms that are taken into account during the combination into the final angiogram (e.g. step 6) is reduced, and so the quality of the final angiogram may be degraded. This problem can be dealt with by increasing the number of repetitions of the B-scans. However, such an approach increases the acquisition time, and extends the time for the execution of the OCTA process. Thus, this is not an efficient way of dealing with the problems encountered in method B.
Non-patent literature 3 discusses another known technique in structural OCT imaging where a single shift between adjacent B-scans is determined and corrected. However, such method is not efficient enough to entirely remove the artefacts in OCTA imaging as a result of eye movements.
Patent publication number WO2006/077107 discusses a technique that tries to solve the problem of rapid eye movements during long acquisition of data by collecting at least two acquisitions of data sets in an orthogonal direction. Based on a similarity function, it combines both data sets into a single data set free of movements. This method can efficiently reduce the influence of rapid eye movements which creates the distortion of two-dimensional scanning positions. However, compensation of head-induced influence is not discussed in detail. Moreover, the method is not suitable enough to align the B-scan images at the same position of the eye.
Therefore, an approach to reduce the influence of eye movement during OCTA measurement is desired in order to maintain high quality OCTA images. In particular, there is a need for reducing the influence of eye movement caused by head-induced movements of a patient during OCTA measurement. Moreover, an approach to reduce the influence of eye movement not only during OCTA measurement but also during OCT measurement is desired.